1. Field of the Invention
The present invention relates to an apparatus for measuring an in-plane high frequency magnetic field generated from a microwave-assisted magnetic head.
2. Description of Conventional Art
Conventionally, improvements in a longitudinal recording density for a magnetic recording medium, such as a hard disk, have been attempted by making magnetic particles that form a magnetic recording layer minute, improving the material, and micro-structuring the magnetic head. In addition, a perpendicular recording type magnetic recording and reproducing device is being utilized recently in which further improvements in the surface recording density have been attempted by magnetizing a recording layer in a direction perpendicular to a surface of the magnetic recording medium. Yet further improvements in the longitudinal recording density are expected in the future.
In contrast, a thermal fluctuation easily occurs in recording magnetization as a result of making recording bits and magnetic particles minute. Therefore, it is preferable that the magnetic particles that have large magnetic anisotropy energy Ku (magnetic anisotropy magnetic field Hk) and a large coercive force Hc are used for the material for the recording layer.
However, when the magnetic particles with large magnetic anisotropy energy Ku are used for the material for the recording layer, the coercive force Hc of the recording layer becomes a large value, such as 4 KOe. It is said that a recording magnetic field that is more than twice of the coercive force is generally needed for saturation magnetic recording. Therefore, with the performance of the conventional magnetic head, it has been occasionally extremely difficult to saturation-magnetize the recording layer. That is, there have been cases where recording and deleting magnetic data are difficult.
Data is magnetically recorded on a magnetic recording medium by applying electric current to a primary coil positioned adjacent to a main magnetic pole of the magnetic head and using a perpendicular recording magnetic field that is generated from a front end of the main magnetic pole. It has been considered to dramatically reduce the perpendicular recording magnetic field needed for such a magnetic reversal by superposing an alternating current (AC) magnetic field in an in-plane direction in a microwave band that is equivalent to, or close to, a ferromagnetic resonance frequency of the recording medium onto a perpendicular recording magnetic field that induces the magnetic reversal. This assisted recording method is called a microwave-assisted magnetic recording (MAMR), and effects have been verified through experiments.
Mainly two types of MAMR have been proposed. One type is a type that generates a microwave magnetic field in the in-plane direction by forming a spin torque oscillator (STO) formed of a multilayer magnetic thin film in a gap between the main magnetic pole and an auxiliary magnetic pole of the recording head, and by driving a bias electric current to oscillate the STO, as discussed in Document 1 (J. Zhu et al.; IEEE Transaction on Magnetics, Vol. 44, No. 1, p. 125) (this may be called a self-excited type).
Another type is a type that provides a secondary coil in, or adjacent to, the gap between the main magnetic pole and the auxiliary magnetic pole of the magnetic head and that drives a microwave band AC to the secondary coil to generate the in-plane AC magnetic field, as discussed in Document 2 (US Patent Application Publication No. 2007-253106) (this may be called an induced type).
In consideration of mass production and utilization of such a magnetic head, to secure the reliability of the device, the in-plane high frequency magnetic field intensity generated by the microwave-assisted magnetic head must be precisely measured for each magnetic head, and a highly sensitive and low-cost property measurement apparatus must be developed.
However, the development of the device has the following significant technical issues:
(1) For both the self-excited type and the induced type, the gap between, or adjacent to, the main magnetic pole and the auxiliary magnetic pole that generates the in-plane high frequency magnetic field is assumed to be, at most, approximately 30 nm. Therefore, the in-plane high-frequency magnetic field is generated from an extremely minute region;
(2) To express the effects of microwave assistance, a strong in-plane high frequency magnetic field of 2 KOe or greater, for example, is required, and such in-plane high frequency magnetic field must be present; and
(3) The frequency of the in-plane high frequency magnetic field is equal to, or close to, the ferromagnetic resonance frequency of the recording layer of the magnetic recording medium that is subjected to recording, and is generally approximately 10 GHz to 40 GHz, which is extremely high.
In contrast, as a method for measuring the recording magnetic field of a conventional longitudinal recording magnetic head, there has been proposed a method for measuring the recording magnetic field by positioning a magnetic sensor, more specifically a giant magnetoresistive (GMR) head, facing a flying surface of the magnetic head (JP Patent Application Laid-Open Publication No. 2009-301610).
However, the method proposed by the above publication suggests that the frequency of the recording drive current is approximately 20-700 MHz. This frequency is totally different from the frequency band of 10 GHz to 40 GHz, which is the frequency expressed by the microwave-assisted magnetic head.
Further, because an element resistance is low and the output is small when a GMR head is used as the measurement sensor, it is extremely difficult to conduct a reliable measurement simply by adjacently positioning the sensor since the signal-noise (S/N) ratio for the measurement signal cannot be securely set.
The present invention was created based on such facts. An object of the present invention is to provide a new measurement apparatus that allows precise measurement of the in-plane high frequency magnetic field generated by the microwave-assisted magnetic head with high reliability.
Such measurement apparatus can secure high density recording and improvements in recording quality and contribute to a succinct shipping inspection, a low cost and a high throughput.